Turbine rotors include a peripheral array of individually manufactured blades mounted to the rotor hub with interlocking grooves and blade roots. The blades must be securely mounted to withstand high rotational speeds, axial loading of gas flow and high temperature variations during operation. However, since blades are periodically removed during repairs and inspection, blade locking mechanisms ideally should be capable of rapid removal and reuse with minimal damage to the hub and blade root.
Various locking devices have been proposed and used in the prior art. In many cases the locking mechanisms require their own machined and accurately fitting parts which add to complication of manufacturing and cost. In some cases, the prior art locking devices include machined notches or other structural features which can result in stress concentrations in components which are already highly stressed. In addition, some prior art blade locking devices and methods include use of parts such as rivets which must be removed and discarded during blade replacement operations. Removal risks damaging of adjacent hub and blade surfaces, and as well increases the overall cost of the fastening system by requiring skilled labour for installation and inspection as well as by producing scrap.
Examples of common rotor blade locking devices are found in U.S. Pat. No. 2,761,648 to Purvis et al. and U.S. Pat. No. 3,598,503 to Muller. Both of these systems use a bent sheet metal bar inserted between the blade root and radially inward floor of the machined groove in the rotor hub to positively engage both components together. Other examples include a leaf-spring locking bar with mating groove in the blade root as shown in U.S. Pat. No. 2,847,187 to Murphy, and U.S. Pat. No. 5,518,369 to Modafferi which utilizes a simple bent bar disposed between the rotor hub and the underside of blade platforms.
A significant disadvantage of the above examples of the prior art is that a single component is used to resist the forces that would dislodge the blade from the root hub and for locking the blade to the hub itself. In general, the locking function is performed by bending a sheet metal bar for example.
Utilizing a single component introduces a trade-off between a desire for high strength and for high ductility. In general, high strength metals tend to be more brittle or less ductile, while highly ductile metals are of relatively low strength. High strength materials have the advantage of reduced weight and size, which is especially critical in aircraft design. Blade locking mechanisms that include bent or deformed connectors require sufficient ductility to be bent during installation. Materials which are ductile and easily bent are generally of much lower yield strength than hardened or heat treated materials which will have a high strength to weight ratio and can be used for efficient resisting of high forces.
Further disadvantages of prior art systems are that the blade is often not restrained on both sides of the rotor hub and that the amount of bearing area available for resisting forces is limited.
A further disadvantage of prior art systems is that bending bars raises concern over fatigue stress cracking in the bent area. Repeated bending during multiple installation and removal increases the likelihood of metal fatigue failure. Disposal of aircraft engine quality materials is highly undesirable and expensive, however, if bent bars are used to resist forces between the hub and blade, repeated use of the bars is not practical.
As an object of the present invention therefore, to provide an easily manufactured and installed blade locking mechanism which fully restrains the blade in both directions axially and does not introduce undesirable waste of material, metal fatigue concerns or risk of stress concentration.